Microwaves can be really useful, but there are some things you should never put in one. Certain materials and foods can cause sparks, and microwaving can leach chemicals into your food and even set off explosions. Here are five things you should always keep out of the microwave.

Metal

You've probably heard never to put metal in the microwave, but do you know why? Metal reflects microwaves.

If you cover your food with aluminum foil, for instance, the microwave will bounce off the foil and your food won't warm. The reflected waves can also damage your microwave. Items with pointed ends, like forks, can produce a concentrated electric field and create sparks, according to MIT.

Eggs in the shell

Microwaves cook very quickly. So quickly, in fact, that the steam inside the egg builds up too rapidly. Eventually, too much steam builds up and the egg explodes.

Some plastics

When microwaved, some plastics leach BPA and phthalates into foods. These chemicals are believed to be "endocrine disruptors." Many containers used to package foods, such as butter containers and yogurt tubs, are not microwavable. Always look for a microwave-safe label before popping a container in to nuke.

Plastic bags

You should never microwave food inside plastic bags. Steam can build up in bags, causing them to explode. The bags also have the potential to melt.

Vintage plates and mugs

According to the Smithsonian, microwaving in old plates and mugs that are over 40 years old can cause lead from the glaze to seep into your foods. You can test your old tableware with lead testing kits to make sure this isn't a threat.

Editors' note: This article was originally published Dec. 18, 2015 and has been updated.